The present invention relates to a biasing force application means for biasing a reel shaft in a direction for winding up a webbing in a seat belt retractor employed in a seat belt system which is provided on a vehicle, for example, an automobile, for the purpose of protecting an occupant when an emergency situation occurs by locking the reel shaft from rotating to thereby prevent the webbing from being pulled out. More particularly, the present invention relates to a device for maintaining a biasing state of the biasing force application means that is formed as a subassembly at the time of assembling the seat belt retractor.
Automobiles and other vehicles have formerly been equipped with seat belt system for protecting occupants. In general, a seat belt system has a seat belt retractor. The seat belt retractor has a reel shaft for winding up a webbing. The reel shaft is constantly biased in a direction for winding up the webbing by a relatively small biasing force from a biasing force application means having a spring.
Accordingly, in a typical seat belt retractor of the type described above, the webbing is wound up on the reel shaft under normal circumstances, and when it is necessary to unwind the webbing in order to fasten the seat belt, for example, the webbing can be unwound as desired with a relatively small pulling force. At the time of occurrence of such an emergency situation that a high degree of deceleration acts on the vehicle, the reel shaft is locked from rotating to thereby prevent the webbing from being pulled out by the inertial movement of the occupant's body.
In general, the biasing force application means, which constantly biases the reel shaft, is mounted on the outer surface of one side wall of a U-shaped frame for rotatably supporting the reel shaft, and it comprises a power spring, which is a spiral spring for producing biasing force, a bush which is engaged with the inner end of the power spring and transmits the biasing force of the power spring to the reel shaft, a spring casing which accommodates the power spring and the bush and which is engaged with the outer end of the power spring, and a cover which covers the spring casing.
Incidentally, when the seat belt retractor is to be assembled, the biasing force application means is usually assembled in advance as a subassembly, and this subassembly is attached to the frame. When the biasing force application means is in the subassembled state, it is necessary to apply the biasing force of the spring to the reel shaft in the webbing winding direction at all times. Therefore, the power spring needs to be held in a state where it has been wound up by a predetermined amount in the webbing unwinding direction. Accordingly, a device for maintaining the biasing state of the biasing force application means has heretofore been employed when the seat belt retractor is assembled.
In the conventional biasing state maintaining device for the biasing force application means, a relatively large hole is opened in a portion of the cover that faces the bush, and in a state where the spring casing, the bush, the power spring and the casing have been assembled with the power spring wound up by a predetermined amount in the webbing unwinding direction, a stopper is inserted into the hole in the cover and engaged with the bush and the spring casing so as to prevent rotation of the bush in at least a direction in which the power spring is unwound, thereby holding the power spring in the wound-up state.
Then, the subassembled biasing force application means is mounted on the frame with the bush fitted to the rotating shaft of the reel shaft so that the bush is unable to rotate relative to the rotating shaft. Thereafter, the stopper is removed, and the hole in the cover is closed with a lid. Thus, the assembling of the biasing force application means is completed.
However, with the conventional biasing state maintaining device for the biasing force application means, the stopper must be engaged with the bush and the spring casing. Therefore, the assembling operation is relatively complicated and requires time and labor. In addition, there is likelihood that the stopper will disengage and fall from the hole of the casing while the subassembled biasing force application means is being transported to the seat belt retractor assembly line. Thus, it is relatively difficult to maintain the biasing state of the biasing force application means.
Further, it is necessary to carry out the operation of removing the stopper and closing the hole of the cover with the lid after the subassembly has been mounted on the frame. Thus, the prior art requires a large number of parts as well as a large number of manufacturing operation steps.